ntp: smear leap second with slewing mode and smoothing

Suppress leap second in packets sent to clients when smoothing and leap
second slew mode are enabled.

reference.c

@@ -328,6 +328,14 @@ REF_SetModeEndHandler(REF_ModeEndHandler handler)
 
 /* ================================================== */
 
+REF_LeapMode
+REF_GetLeapMode(void)
+{
+  return leap_mode;
+}
+
+/* ================================================== */
+
 static double
 Sqr(double x)
 {